Purchasing PVP gear at 90
In 5.2, the PvP gear system is kind of being re-done.
Right now, there's level 450 Contender gear, then 458 Dreadful gear, and 483 Malevolent gear. And all of them can be upgraded at least once (Malevolent twice), I think.
5.2 will feature the removal of item upgrading. So, if you were to upgrade a Malevolent piece to 491, it will be better than 5.2 Malevolent gear. BUT I don't know whether it will be better than Tyrannical or not.
In 5.2, Dreadful will basically be the new Contender. It will be craftable.
Malevolent gear will basically be the new Dreadful, as it will be purchasable with Honor points.
Tyrannical will be the new Malevolent, purchasable with Conquest points. And, once a player earns 27,000 Conquest points in Season 12 (current), they will be able to purchase Tyrannical with Honor points (but 27k is enough to buy one full set of Tyrannical. The switch to Honor is for alternate specs, I think).
I have no idea when 5.2 will be released.
As of right now, Conquest weapons cannot be purchased until a player earns 7,250 conquest points. Which means that by the time (now) you can get a Malevolent weapon, you'd also have enough to buy 2 - 3 pieces of Conquest armor beforehand.
I think the current best way to buy gear is Helm first, because of the Meta gem slot (I could be wrong, though). However, if you don't want to drop several-hundred gold on a Meta gem for your Helm, then I'd go with the Chest.
In general, whichever will give you the most beneficial increase of stats.
I went for Dreadful Chest first, because I had a 471 PvE heroic helm that I had Meta-gemmed already. So I wasn't in desperate need for a good helm.
Then I got Dreadful shoulders, legs, and gloves.
By the time I got nearly full Dreadful gear, I was able to purchase a Malevolent Chestguard.
So, there's a lot of variables involved in maximing your "bang for your buck".
It will depend on how long until 5.2 is released, if Conquest and Honor points will be transferred or converted to gold, and the strength of the New conquest gear. All of which I don't know, but there might be information out there.
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